Parametric receiver input circuits, i.e. circuits with controlled susceptance, utilizing, for example, varactor diodes, are known to be superior in sensitivity to conventional receivers which employ converters with controlled conductance. Among the multitude of prior art variations of parametric circuits for receiver input stages, the following examples merit particular emphasis in connection with a coherent phase down converter:
1. Lower sideband down converters with AM or FM modulated pump source;
2. Nonreciprocal degenerated converter cascade amplifiers;
3. Upper sideband down converters with high conversion ratio and termination at the image frequency by a real resistance.
These examples are distinguished either by very low system noise temperature as in the case of example 3, above, or by an improved signal to noise ratio, as in the case of examples 1 and 2, above.
The mechanisms giving rise to these properties are already very well known. In particular, for systems according to examples 1 and 2, the term noise figure has to be defined in a more generalized way.
The achievement of the signal to noise ratio improvement (systems with F&lt;1) can be explained with the aid of a generalized definition, as described in U.S. Pat. No. 3,911,365. Conventional systems are also included in the definition.
In the treatment of these types of receivers, it becomes evident that two characteristic properties are the major contributors to an improvement in the signal to noise ratio: firstly, the possibility of reducing the system bandwidth from the HF to the IF region, e.g. using a feedback connected FM receiver; and secondly a different amplification of the signal and noise energy, e.g. using a feedback connected AM receiver or converter cascade amplifier.
For system considerations in the microwave range it may be a drawback, however, for the first converter in the converter cascade amplifier to be an up converter, i.e. to be operated with a high pump frequency. The lower sideband down converter inverts the signal spectrum and can therefore not be used without further lower sideband conversions.